When a plant photosynthesizes, the plant utilizes optical energy such as solar light so that the plant generates carbon hydride compounds such as glucose (i.e., C6H12O6) including hydrogen and carbon in addition to oxygen (i.e., O2) from water (i.e., H2O) and carbon oxide (i.e., CO2) absorbed in the plant. A system has been studied such that the photonic synthesis is artificially performed using semiconductor photocatalyst, hydrogen (H2) and oxygen are generated from water using optical energy of the solar light, generated hydrogen is stored, and the stored hydrogen is used by a fuel cell to generate electricity so that electric energy is retrieved if necessary.
For example, as shown in FIG. 11, the semiconductor photocatalysts J2, J3 for providing the anode electrode and the cathode electrode are arranged in a casing J1. Thus, the sun light is irradiated so that the optical energy is supplied. Thus, a reaction shown in chemical equation E1 occurs at the semiconductor photocatalyst J2 as the anode electrode, and a reaction shown in chemical equation E2 occurs at the semiconductor photocatalyst J3 as the cathode electrode. Thus, the hydrogen and oxygen are generated. Here, the term “h” in the chemical equation 1 represents a hole, and the term “e” represents an electron.2H2O+4h+→O2+4H++2e−  (E1)4H++2e−→H2  (E2)
Specifically, a titanium dioxide electrode (i.e., TiO2 electrode) and a platinum electrode (i.e., Pt electrode) as the semiconductor photocatalysts J2, J3 are located in water. When the light is irradiated on the titanium dioxide electrode, water is decomposed so that the oxygen is generated at the titanium dioxide electrode, and the hydrogen is generated at the platinum electrode. Further, current flows between the titanium dioxide electrode and the platinum electrode. These are defined as Honda-Fujishima effect.
Since water is generated when a fuel battery generates electricity, it is possible to provide a recycling-oriented regenerative energy generating system using water and sun light without fossil fuel by recycling water into artificial photonic synthesis. Further, fuel may be synthesized by absorbing carbon dioxide (CO2).
However, in the artificial photo synthesis provided by the Honda-Fujiyama effect, the efficiency of the photo synthesis is not high because the titanium dioxide can absorb only ultra-violet light in the sun light.
It is necessary to satisfy the following three conditions in order to perform the artificial photonic synthesis with high efficiency. The three conditions will be explained with reference to FIGS. 12 to 14.
The first condition is such that the light energy is absorbed in a visible light range. FIG. 12 shows an energy band view of a general semiconductor. A band gap in FIG. 12 is defined as a difference of an energy level between an upper level of a valence band and a lower level of a conduction band. The absorption of the sun light depends on the band gap. Specifically, only the light energy equal to or higher than the band gap of the semiconductor is absorbed. For example, the relationship between the wavelength of the sun light and the light intensity is shown in FIG. 13. Here, the wavelength range of the visible light is in a range between 400 nm and 800 nm. The band gap of the titanium dioxide is 3.2 eV. Thus, only the light energy is absorbed in the wavelength range equal to or shorter than 400 nm, which generates the light energy corresponding to the band gap of 3.2 eV. The energy density of the light energy is high in the visible light range. Since the titanium dioxide cannot absorb the light energy having the visible light range, the efficiency of the photonic synthesis is not high. Thus, when the light energy is effectively absorbed in the visible light range, it is necessary to narrow the band gap, and this is the first condition.
The second and third conditions are such that requirements for generating oxygen and hydrogen from water are satisfied. As shown in FIG. 14, the oxidation electric potential of water required for generating oxygen by oxidizing water using a hole is 1.23 V with reference to the standard hydrogen electrode electric potential (i.e., SHE) as a standard. Unless the electric potential (i.e., an upper band energy level) of the upper level of the valence band is disposed on a positive side (i.e., a lower side) from the oxidization electric potential of water, the oxygen is not generated from water using the hole. Further, as shown in FIG. 14, the hydrogen reduction electric potential required for generating hydrogen by reducing the hydrogen ion using the electron is 0 V. Unless the electric potential (i.e., a lower band energy level) of the lower level of the conduction band is disposed on the negative side (i.e., the upper side) from the hydrogen reduction electric potential, the hydrogen is not generated using the electron. In order to satisfy both conditions, it is necessary to have a large band gap.
Thus, the first condition is opposite to the second and third conditions. In order to perform the photonic synthesis effectively, it is important to valance these conditions.
Alternatively, the artificial photonic synthesis, which is different from the synthesis provided by the Honda-Fujishima effect, may be performed using the semiconductor photocatalyst. For example, the semiconductor photocatalyst is proposed in Patent document No. 1 such that the anode electrode has a structure that an aluminum gallium nitride layer (i.e., AlGaN layer) is arranged on a gallium nitride layer (i.e., GaN layer), and the cathode electrode is made of metallic material including platinum mainly.
Further, another semiconductor photocatalyst is proposed in Patent document No. 2 such that water absorption and water oxidation (for generating oxygen) are performed using tungsten oxide (WO3) and bismuth vanadate (BiVO4), and the light absorption and the hydrogen reduction (for generating hydrogen) are performed using PT/SrTiO3. Specifically, generation of oxygen and generation of hydrogen are performed using different materials in two steps. Two reactions are continuously performed using electron transmission material.
However, the semiconductor photocatalyst described in the Patent document No. 1 provides to satisfy the requirement for the oxidation electric potential of water and the reduction electric potential of hydrogen, but the photocatalyst can absorb only the light having the wavelength equal to or shorter than 350 nm, so that the light energy equal to or less than a few percent of the sun light is absorbed.
On the other hand, in case of the semiconductor catalyst for performing the generation of oxygen and the generation of hydrogen using different materials in two steps, the requirements for the oxidation electric potential of water and for the reduction electric potential of hydrogen are satisfied using different materials, and further, it is possible to narrow the band gap of each material. Thus, the absorption efficiency of the light energy is improved. However, it is necessary to arrange Fe2+ and Fe3+ in order to continuously perform each reaction, and the wavelength of the light, which can be absorbed, is equal to or shorter than 500 nm. Although the wavelength range of the light to be absorbed is expanded, only 20% of the light energy of the sun light is absorbed.
Patent document No. 1: JP-2013-49891 A
Patent document No. 2: JP-2005-199187 A